Update: this answer is no longer applicable as of 2017. See https://blog.google/products/search/improvements-searching-special-characters-programming-languages/
Google strips most punctuation from queries, as described here, so it won’t help you with the bash syntax.
It’s very easy to search for the string “##” in the bash documentation: Just run “info bash”, hit “s”, and enter “##” as the search string.